Telegraph switching system



2 Sheets-Sheet 1 Dec. 6, 1949 o. BLYHOLDER TELEGRAPH SWITCHING SYSTEMOriginal Filed Aug. 14, 1944 -35 hows Dec. 6, 1949 o. BLYHOLDERTELEGRAPH SWITCHING SYSTEM Original Filed Aug. 14, 1944 2 Sheets-Sheet 2V MRER \Uh\s is! u Patented Dec. 6, 1 949 UNITED STATES PATEYNTOFFICETELEGRAPH swrronme SYSTEM Orlando Blyholder, Chicago, Ill., assignor toKellogg Switchboard and Supply Company, Chicago, Ill., a corporation ofIllinois Original application August .14, 1944, Serial No. 549,339, nowPatent No. 2,464,371, dated March 3 Claims.

GENERAL DESCRIPTION I The invention is illustrated as embodied in atelegraph switching system employing the socalled hub repeatingarrangement wherein two or more lines may be interconnected through acommon hub conductor having individual spoke conductors for therespective interconnected lines, and in which a two-way tele-. graphrepeater is interposed between each individual line and its associatedspoke conductor.

A specific object is to provide a telegraph switching system in whichthe lines have ordinary appearances at the switchboard and in whichreliable provisions are made for signaling the switchboard operator overany line when connection or disconnection is desired.

Other objects and features of the invention, more or less incidental tothe foregoing, will appear hereinafter.

THE DRAWING The accompanying drawings, comprising Figs. 1 to 3, showsufiicient of the apparatus incorporated in a system embodying theinvention to enable the invention to be understood.

Fig. 1 shows one embodiment of the invention;

Fig. 2 shows a modification of the equipment associated with line LI ofFig. 1; and

Fig. 3 shows a second modification of such equipment.

Figure 1 Fig. 1 shows three subscriber lines of a system including anydesired number of such lines; first and second lines LI and L2, and thelast line LL. These lines serve stations SI, S2, and SL respectively,shown diagrammatically as teletypewriter stations. The illustrated linesare provided with similar two-way repeaters RI, R2, and RL respectively,repeater RI being shown in full. Spoke conductors SPI, SP2, and SPLconnect the illustrated repeaters respectively with their switchboardjacks, J I, J2, and JL, with which the jacks J I J2, and JL', arerespectively multipled, enabling patching cords such as PCI, PC2, andPC3 to be used to interconnect the spoke Divided and this 24, 1945,Serial No. 612,499

application August conductors in any desired arrangement of pairs' andhubs. 7

Each repeater contains a receiving relay, such as RR of. repeaterRI. Thereceiving relay RR may be of any desired form of polarized relay,

being illustrated as of the type in which the polarized armature passesaxially through the winding space. Such a relay may be adjusted toassume a neutral position with no current flowing through any of itsthree windings. The armature of relay RR is normally held in markingposition (against contact M) by the right-- hand or marking winding,energized in a circuit including the positive pole of the current supplyand the associated regulating resistor. The right-handor marking windingmay be eliminated if relay BB is biased to marking position, such asmagnetically or by spring action. The circuit of line .LI includes thetwo conductors thereof in series withthe equipment at station SI. It maybe considered as originating at the positive pole of the current-supplysource, and the normal currenteflow thereover is controlled by theassociated adjustable resistor. turn path for line current is throughthe lefthand or line winding of receiving relay RR, and

the plate of sending tube ST, to ground through the grounded cathodeelement thereof. Assuming that .the three windings of relay RR are,

equivalent, the line current is preferably regulated to be about doublethe current in the righthand or marking winding, and is in such a direction through the line winding as to assist the marking winding inholding the armature of the relay in marking condition, as is indicatedby the fact that the vertical arrows on the windings in question aredirected downwardly.

The magnetic efiect of the current through the left-hand or line windingof receiving relay RR is neutralized by the opposed current flow of subjficial-line circuit, which includes the associated adjustable resistorconnected between such winding and the positive pole of the supplysource.

The return path for the artificial line is through the plate of thebalance tube BT, to ground through the grounded cathode element thereof.

Each of the tubes ST and ET is indicated as a tetrode, having itscontrol and screen grids in Each screen grid the usual relativepositions. is supplied with, positive potential from the resistor. Thecontrol'grids of tubes ST and BT The reare both connected to the markingcontact M of receiving relay RR, and are normally supplied with groundpotential through the associated grid resistor GR, permitting a normalflow of current through both. A tube such as the commercially available6L6 beam-power tube 'is satisfactory at ST and BT, using a positivesupplypotential'of 250 volts, for example.

Lines Ll to LL are provided with line circuits" L! to LCL respectively,of which the-line'cir cuit LCI is shown in full. Line circuit LC! iscontrolled over the associated spoke-conducton SPI through the grid oflinetubeLT. Tube;'LT,' as well as the other triodes shown herein; may"be of the commercially availablex6051typetusing the same positive supplypotential asthe sending and balance tubes.

The purpose of line circuit LCI, is to light call' lamp Cl at theswitchboard responsive to-:a prolonged opening of line Ll (as by breakBKI) in excess of one second, for example, and to light the disconnectlamp Dfresponsive to a break-in theassociated line in excess of.fiveseconds, for

example, at a time when aconnection with the line exists through theswitchboardi. Line circuit LCl includesline relay-{normally operatedthrough the plate-cathode circuit of line tube LT by half-wave-rectifiedcur-rentfromrelay terminal R, supplied with alternating; current ofcommercial frequency and suitable voltage Relay 2 is arranged tocontrolthe potential of the grid of timing tube TT by. normally.maintaining a cutoff'bias on such-grid,.acro;-is condenser 6, and bydischarging this biasing potentialthrough resistor I for a one-seconddelay,.andthrough re-' sistors l and 8"for a five-second-de-lay. As willhereinafter appear, timingtubefTT controls one w or another of the calland disconnect relays: and4, accordin to the operated-orrestoredtcondition of transfer relay '3-,-.controlled.1from either oftheassociated jacks Jl andiJ-l'. Relays3 to 5 are supplied-with currentfrom the associated relay terminals R.- Each such relayiis preferably ashaded-pole relay which remains operated steadily when energized-by-alternatingf pulsating,

or direct current.

The switchboard operator is supplied witha telegraph station si alsoillustra ted as a teletypewriter station, controlled overthe operator'slineLU. This line-terminates inthe operator's repeater R0, exactlysimilar to repeater RI. Re-- peater R0 is connected with the operatorsswitchboard jack'Jfl by'the spokecon'ductor SPO.-

In Figure 2, the equipment ofaline Ll oflFig. 1. ismodified by replacinglinetube- LT and-relay 2 of line circuit-LCI by relay 2'-A' ofline-circuit LCl-A. Line circuit LCl"Af-isarranged torespond only tobreaks occurring'in the associated line and is independent of'control'over'th'e common hub by way of the associated spoke conductorSP1, as will appear hereinafter. For this purpose, relay 2A isadouble-wound differential relay, having. one'winding in series with lineLI and the other winding inlseries'lwith the artifi'cial or balance lineof the repeater RI. In the arrangement of Fig. 2, the transmission of -adisconnect signal from a connected line causes the lighting of thedisconnect lamp associated with that line only, whereas in Fig. 1. thedisconnect lampslight at all interconnected lines. responsive toadisconnect break at anyone of them.

Figure 3 Fig. 3 shows a further modification of the equipment of line Li(Fig. 1), illustrating what may be termed a uniform-current system. Thearrangement in Fig. 3 is such that current flows alternately inthesubscriber'line and in the associated artificial or balance line,.thereby simplifying the power-supply requirements, in addition tohalving the simultaneous or demand current for the several natural and.artificial lines, as will be explained more indetail hereinafter. Fig. 3cmploys the'additional tube PI, used as a phase-inverter triodeeffective to transmit a potential of the opposite sign to the controlgrid of balance tube BT'when a'potential of a given sign is impressed onthecontrol grid of the sending tube ST.

DETAILED DESCRIPTION The disclosure havingbeen described generally,..adetailed'description of the operationoi the system willnow be given.

A. FIGURE 1 The system of "Fig." 1* will be considered first.

A1. Calling the operator Assuming that the-subscriber at'substatlon Sl(Fig. 1) desires a: connection,- he opens the associated line Li, as bybreak key BKI, for an interval somewhat" in excessof'one second, andthen reclosestheline. W'henathe' line is opened, the flow'of linecurrent ceases, deenergizing the left-hand orline winding of'receivingrelay RR.

Tliereupon, relay RR operatesits armature fromengagement with itsmarking contact M into engagement with its'spacing contact S; by virtueof "the fact that the more powerful current in-the -middle or balancewinding then overcomes the current in the "right-hand or markingwinding. Spacing current from the negative biasing terminalisthereby'impressedon the associated spoke conductor SPI.

5 Since spoke conductorSPFis normally disconnected at the jacks J I andJ l", the placing of the negative spacing'potential thereon has noeffect except in the line circuit LCl, where it blocks current flowthrough line tube LT. The normally operated line relay 2 thereupon'restores, disconnecting the grid of timer tube TT (and the upperterminal-of timingcon'denser 6) from the source of biasing potential andconnects it through the adjustable one-second -resistor l, and contacts3 of transfer relay =3,- to ground;- At the end-of about one second, thenormal negative charge of grid condenser 6 sufficiently dissipatedthrough resistor l to renderthe control grid of timing tube TTsufficiently positive to enable plate current of relay-operating valueto flow-therethrough. Call relay 5 thereupon operates in a circuit fromground, through the. cathode and plate of tube -TT,' armature l oftransfer relay 3 and its back contact, and thence through the winding,of call relay 5 'to relay terminal R, supplied With alternatingpotential. The current which flows through relay 5 at this time ispulsatingbecause of the half-Wave rectifying action of tube TT; Uponoperating, relay 5 closes alocal self-locking circuit-at its contacts I,to ground through the back contact of armature 2 of transfer re1ay3;This self-locking circuit may include the illustrated current-limitingresistor. At its contacts 2', relay 5' closes a lighting circuit forcall lamp Cl.

When the circuit of line Ll is reclosed, the resumption of current flowthrough the line winding of receiving relay RR. again neutralizes theeffect of current flow through the balance winding of the relay,permitting the right-hand or bias winding of the relay to return thearmature from spacing position to marking position. When this occurs,positive (ground) potential, obtained through grid resistor GR in therepeater RI is again applied to the grid of line tube LT in line circuitLCI, whereupon current flow is resumed through tube LT and line relay 2.Relay 2 thereupon reoperates and disconnects the grid of timing tube TTfrom ground (cathode) potential by way of resistor 1 and transfers it tonegative biasing potential. When this occurs, condenser 6 again chargesto a negative value and the grid of tube TT becomes negative,terminating the flow of plate current therethrough. The resistorincluded in the lead extending to the front contact of relay 2 is toprevent momentary lowering of the biasing potential on the common supplylead by a sudden inrush of charging current to condenser 6. Thecessation of the flow of plate current through tube TT does not causecall relay 5 to restore because of the above-noted selflocking circuitof relay 5 through its contacts I, Only alternating current now flowsthrough the winding of relay 5, but this relay is able to remainoperated steadily when supplied only with alternating current, aspreviously noted.

A2. Answering Upon noting the lighted condition of call lamp Cl, theoperator may insert one plug of an idle patching cord, such as PC3, intothe jack J l (or the jack J l) and insert the other plug of suchpatching cord into the operators jack J 0. By this operation, spokeconductor SP! is connected electrically with spoke conductor SPO, theconcerned single-conductor patching cord serving as the common hubbetween the two spokes. With the repeaters RI and R both in markingcondition, both spokes are at ground potential, wherefore there is noimmediate interchange of current over the interconnection.

As a result of the insertion of a connecting plug into either of thejacks J I or J l', the upper contacts of such jack connect ground to thelower terminal of transfer relay 3, operating relay 3 in an obviouscircuit. Armature I of relay 3 disconnects the plate of tube 'I'T fromcall relay 5 and transfers it to relay 4. Armature 2 of relay 3disconnects ground from the self-locking circuit of call relay 5 andprepares a self-locking circuit for disconnect relay 4. Call relay 5 nowrestores, thereby extinguishing call lamp C'l. Contacts 3 of transferrelay 3 remove the shunt normally existing around resistor 8 so as toincrease the effective discharge time of grid condenser 6 from about onesecond to about five seconds, thereby insuring that no likelycombination of telegraph signals nor a break-in signal will result in apremature operation of disconnect relay 4.

Having thus interconnected operator line L0 with the calling subscriberline Ll, the operator may now type out the usual challenge at theoperator station S0 to inform the subscriber at station SI that he is inconnection with the operator, the word operator, for example. By thisoperation, line L6 is opened and closed, as required. Each time the lineis opened, the cessation of current in the line winding of repeatingrelay RRU causes the armature of such relay to move from its illustratedmarking position into engagement with its spacing contact, therebyimpressing negative potential on spoke conducto r SPO Each time linecurrent is resumed in the line L0, the armature of relay RRU isreoperated from spacing position to marking position, therebydisconnecting the negative spacing potential and reconnecting conductorSP0 to the grids of tubes STE} and BTU. At the end of the instantsending operation, relay RRll comes to rest in its illustrated markingposition, in readiness for the receipt of instruction from the callingsubscriber.

Each time negative spacing potential is placed on conductor SP6 as abovenoted, such potential is transmitted through jack J 0 and the patchingcord in use (P03), to spoke conductor SPI, by wayof jack J l or J I.With the armature of relay RR in marking condition, each application ofspacing current to conductor SPI results in the application of anegative biasing potential to the grids of tubes ST and BT, causing acessation of current through both tubes and consequently through theassociated subscriber line and the artificial line. When this occurs,the armature of relay RR is unaffected because it is still held inmarking position by current flow through its right-hand or biasingwinding. The resulting cessation of current flow over the subscriberline causes the usual response at the calling station SI, whereat theoperators challenge message is typed.

The subscriber at substation sl may now type out the desiredinstructions to the operator. It may be assumed that the callingsubscriber desires that his line be interconnected with lines L2 and LL.In this event, the operator may telegraphically interconnect line L2with line L! through jacks JI and J2, and a cord such as PCI, and maythen employ a patching cord such as PCZ to interconnect line L2 withline LL through jacks J2 and JL. The concerned patching cords nowcomprise the hub of the interconnected system, the spokes being SPI,SP2, SPL, and SP0. It will be understood of course that as many, or asfew, lines as desired may be interconnected with the calling line byusing the above procedure.

Having made the interconnection, the operator may signal the subscriberat substation SE to proceed, as by typing a word, such as start," atstation S0, resulting in the transmission of the correspondingcombination of spacing and marking signals over spoke conductor SP0, andthence by way of patching cord PCS, to spoke conductor SPI. At this timespoke conductors SP2 and SPL are obviously energized by the spacingSignals, giving the same response at all interconnected subscriberlines. The response at repeater RI is as described hereinbefore, thecurrent flow being stopped in each of the tubes ST and ET for theduration of each negative spacing signal.

Having transmitted the proceed signal, the operator may withdraw fromthe interconnection by removing the plugs of patching cord PCS fromjacks J! and J0.

A4. Intercommunz'cation The interconnected stations S l, S2, and SL maynow intercommunicate as desired. On transmission from station 8!receiving relay BR operates to spacing position on each interruption ofline LI and returns to marking position on each reclosure of the line,thereby transmitting negative spacing signals over spoke conductor SPIand thence over the: interconnecting hub (includingtliepatcliingacords)" to the interconnected spoke conductors SPZ'andxSPL. It will beapparent that a very large number of lines may be: thus interconnected?foreffective: intercommunication; without overloading 'thespacingcontact 1 of; any of: th relays such as RR, because the only loadimposed on such spacing contact is that represented by the, current fiowthrough the grid resistors of the repeaters such as Ri and RL,,corresponding to. grid resistor GR of the. repeater R1. grid resistor maybe ontheorder of 100,000.0hms, or higher, up to a substantial fraction of amegohm, as it needs only to pass sufficient current: to insure the quickresumption of positive potential on the control grids of the associatedtubes such as ST and BT upon disconnection of negative spacing.potential from the interconnected spoke and. hub conductors.

During message transmission, line tube LT in each line circuit respondsby terminating the current flow through the associated line relay Z'foreach spacing signal; and by reestabli'shing current flow therethroughfor each marking, signal. For the most part, the spacing signals are tooshort.

for a slow-restoring relay of the nature of'relay 2 to respond.

During transmission, the subscriber at any station may break'in onthetransmittingstation' by opening the break key thereat, such'as BKI, forabout one or two seconds. that station SIv is the transmitting stationand that station S2 is the break-in station. Opening of line L2 causesthe repeater R2 to transmit spacing potential over SP2 to the repeatersinterconnected therewith. This transmission is begun immediately if thebreak occurs during marking condition. If the break occurs duringspacing condition, the receiving relay in'repeater R2 does not responduntil the existing spacing condition is terminated.

In the repeater Bi, no current'can flow through.

either the subscriber line or. the'artificial. line during the time thenegative spacing potential resulting from the break-in operation isimpressed on the control grides of tubes ST and BT, wherefore. theapparatus at the transmitting'stationsl temporarily ceases to function;therebyinforming the subscriber of the break. Heis expected to'ceasetransmission to permit his line L! to" remain closed for the receipt ofa message from the break-in station;

When the break key BKZ at thebreak-in station S2 issubsequentlyreclosed, the consequent resumption of current flowover'line L2' causes the repeater R2 toremove spacing potential from theinterconnected hub system, enablingtransmission to proceed'thereoverfrom any station;

During transmission of the break-in. spacing signal over the hub system,line relays2may'be restored in all line circuits, and start the timing;

operation of each of the associated" timer tubes TT, but this does notcause disconnectsignaling of the operator, for in theconnected'condition'oi any line, the associated transfer'relay 3 is inop erated condition to increase the length of the timing operation to.about five secondsby including the associated resistor 8. in circuitwith timing resistor l, as previously noted.

A5. Disconnect signaling The subscriber at any interconnected'stationcan signal the operator at any time to request that the entireconnection be taken down or that his line be disconnected f'romtheotherline, or

In practice; this It may be assumed scriberandartificials Itmaybenotedthahsinces.

sumed herein to be fiveseconds;

.scriber line.

g?! lines, ofithe' interconnected group. Disconnect signaling isaccomplished by opening-= the-brealc= keysuch as BK! atstation Si for anintervalexceeding the disconnect timing interval, as- Opening ofthe linecauses spacing potential to be placed on the associated spoke conductor,whence it passes through the-hub system to the other spoke con--ductors, causing'a response at the linecircuit= of all connected lines.In the line circuit LCI, the disconnect spacing potential on conductorSPlcauses line tube LT to restore line relay 2; Relay Z'disconnectstheupper-termina1 of timing condensert and the grid of timing tube TTfromnegative charging potential and transfers them to ground throughresistors I and 8 in series- (contacts 2 of transfer relay 3 are open atthistime becauserelay 3 is operated through theupper contacts of J l orJ I At the end of about five seconds; condenser 6- isdischarged sufliciently to permit an operative flow of current through timing tube TT,whereupon relay 4 oper-- ates'in a circuit from the associatedalternating current terminal R, the winding of relay ll, front contactof armature i of relay 3 andthe said armature, plate of tube TT, andthence to groundthrough the grounded cathode of such' tube. At itscontacts 2, relay closes a lighting circuit fordisconnect lamp DI, whileat its contacts I-it l closes a seli-locking circuit to ground throughthe front contact of armature 2 of transfer relay" 3. Accordingly, relay5 remains'operated following reclosure ofthe break key at the signalingstation andthe consequent reoperation of line. relay 2 and cessation ofcurrent flow through" timing tube TT.

Since. the same line-circuit operation occurs at each interconnectedline(each of the concerned disconnect lamps such as D1, D2, and DL becomeslighted at about the same time), itmay,

be necessary for the operator to inquire which station requiresdisconnection, or Whether all stations require disconnection. This maybe accompiished by interconnecting the operators equipment with theinterconnected hub system. as by Way of patching cord P03 and jacks JL"and Jll; Upon. learning. what is required, the operator may temporariiytake down the entire interconnection to restore the line circuits, andthen reestablish such portion of it as may be desired.

Upon removal of the plug from jack J l trans fer relay 3.restores,disconnecting and restoring; disconnect relay 4 to extinguish lamp Dl.Similar results occur at th other line circuits.

B. FIGURE 2 As previously noted, Fig. 2 shows a modifica-- tion'of theequipment associated With line L] of Fig; l, in that diiierential linerelay 2A replaces line tube LT'and line relay 2. One winding of relay 2Ais in series with the associated subscriberiine Li, and one winding isin series with the artificial line. Normally, relay 2A is-not operatedbecause the opposed windings are of substantially equal strength, but itis operated (by cur-rent flow through its lower winding alone)whencurrent flows in the artificial line ata time when no currentisflowing in the associated sub- This condition obtains only when theassociated station Si is transmitting. It does not obtain-when stationSi is receiving, for at' that time tubes ST'and BT are both locked, to:prevent current him through both lines, sub

line relay 2-A is normally restored instead of normally operated, thenegative biasing potential is connected to the back contact of the relayarmature, and the connection to timing resistors 1 and 8 is brought tothe front contact of such armature. In the arrangement of Fig. 2, whenone station of an interconnected pair orgroup transmits a disconnectsignal (a prolonged line break) such signal operates on no line circuitLCIA except the associated one, and consequently results in the lightingof the disconnect lamp, such as DI, only at the station transmitting thedisconnect signal. Except as noted, the line circuit LCl-A is exactlysimilar to the line circuit LCI (Fig. 1) and operation and restorationof relay 2-A produces results as hereinbefore described for restorationand operation of line relay 2. When a disconnect signal is received, itsuffices for the operator to disconnect the line over which it wasreceived, it being unnecessary to disturb the connections to theremaining lines.

C. FIGURE 3 As previously noted, Fig. 3 shows a further modification ofthe equipment of line I (Fig. 1). Line circuit LCIB is exactly similarto line circuit LCIA, except that the two windings of line relay 2-3 areconnected in aiding relationship rather than in opposed relationship,because operating current flows through only one line (subscriber orartificial) at a time in Fig. 3.

Repeater RIB is generally similar to repeater RI of Figs. 1 and 2,except that phase-inverter tube PI has been added, and the grid circuitof balance tube ET is controlled from the plate circuit of tube PIinstead of in parallel with the control grid of sending tube ST. More inparticular, the control grid of tube ST is connected to marking contactM of receiving relay RR, and is supplied with ground potential throughgrid resistor GR, as in Figs. 1 and 2. The position occupied in Figs. 1and 2 by the control grid of balance tube BT is occupied in Fig. 3 bythe grid of phase-inverter PI. Accordingly, current is normally flowingin the platecathode circuit of tube ST and in the platecathod circuit oftube PI. Balance tube BTin Fig. 3 has its control grid normally suppliedwith ne ative biasing potential through grid resistor GR, whereforecurrent flow is normally blocked through the plate-cathode circuit ofbalance tube BT to provide a condition of no current in the associatedartificial line (including the middle winding of relay RR, the lowerwinding of line relay 2B. and the associated adjustable resistor). Itwill be noted that the connecting resistor OR is interconnected betweenthe plate of phase inverter PI and the control grid of balance tubeB'I'. Accordin ly, current flows continuously from the positive plate oftube PI to the negative control grid of balance tube BT. With no signalbeing received over conductor SPI at the repeater RIB. however, the flowof current through resistor OR is very small because the positivevoltage maintained on the right-hand terminal of resistor GR is very lowwith tube PI in its normal conducting condition. Under this condition,the grid resistor GR is of such value that it accepts all of the currentpassing through control resistor CR and still maintains the potential ofthe control grid of tube BT sufficiently negative to substantially blockthe flow of currielnt through tube BT and associated artificial l e.

When transmission occurs from station S I (either to signal theswitchboard operator or to transmit signals to one or moreinterconnected stations) the opening of line Ll terminates the flow ofcurrent therein and through the left-hand winding of relay RR. Sincecurrent is normally not flowing in the artificial line, the only currentflow remaining through the winding of relay RR is the normal currentflow through the right-hand or-bias winding thereof. The current flowthrough this winding is in such a direction as to cause the armature ofrelay RR to move from marking position to spacing position. At thistime. the current flow through tube PI is not affected, wherefore tubeBT remains unaifected.

When the associated line Ll is reclosed, the resultant resumption ofcurrent flow thereover andv through the left-hand winding of receivingrelay RR causes the armature of such relay to return from its spacingcontact to its marking contact, thereby terminating the transmission ofspacing potential over the associated spoke,

conductor SP l.

With receiving relay RR in its illustrated marking position, receipt ofspacing potential inverter triode PI to become negative,therebyterminating the flow of plate current in both tubes. Tube STdirectly terminates the current flow in the associated line Ll, as aspacing signal to station SI. rent flow through the plate-cathodecircuit of phase-inverter PI, the plate element thereof tends to assumethe full positive potential of the supply source, thereby increasing theflow of current from this positive source through control resistor' ORto the grid of balance tube BT. As a result, the potentialdrop acrossgrid resistor GR is increased sufiiciently that the grid terminalthereof' assumes arelatively positive (or cathode) potential, rendering.the plate-cathode circuit of bal-' ing of line. relay 2- -B. Thecessation of current flow in the plate circuit of tube ST and thestarting of the flow of current through the plate circuit of tube BToccur substantially simultaneously, wherefore there is no open interval,when current is flowing through neither. Since the current normallyflowing through the left-hand winding of relay RR is replaced by asubstantially equal current flow through the middle winding thereof, andin the same effective direction, the armature of relay RR remains inengagement with its marking contact throughout the received markingsignal.

When the negative spacing signal on the associated spoke conductor SPIis terminated, current flow through tubes PI and ST is resumed. Currentflow through the plate-cathode circuit of tube ET is terminated as aresult of the lowering of the positive plate potential at tube PI andthe consequent lessening of current flow (in a positive sense) throughthe resistor CR to the control grid of tube BT. Upon the above-notedresumption of normal marking conditions in the repeater RIB, thearmature of relay RR is held in marking position by current flow throughthe left-hand winding thereof instead of by current through the middleWinding thereof.

In the line circuit LCl-B, line relay 2--B is normally held in operatedcondition by current As a result of cessation of curhow through theupper winding thereof, in series with line 1. The front contact ofthe-armature of relay 2-B maintains anegative potential on-the grid ofthe'associated'tube TT and maintains timing condenser'li in chargedcondition. During the reception of a spacing signal at the repeater-R|'B, the cessation of current flow in the upper winding of line relay2B accompanied by the establishment of the current flow through thelowerwinding thereof, in circuit with tube BT and the middle winding of relayRR. Consequently, line relay 2B remainsaoperated during the reception ofspacing signals atthe repeater Ri B and their transmission over line L1to station Si. On the other hand. with the repeater 'Rl'B in its'normalmarking condition, the opening of line Li at station SI pursuant tosignal transmission causes a cessation of current flow through the upperwinding of relay 2-B with no compensating current flow in the lowerwinding thereof, under which condition relay -2-B is currentless for theduration of the transmitted spacing "signal. Berngof the same type ofslow-restoring relay as noted for relays 2 and 2-A (Figs. 1 and 2),relay 2 'B may not respond tonormal transmission. It does howeverrespond to the comparatively prolonged line breaks at hreak key B'K'l ofstation 181, :as when the associated --line is opened fora periodsomewhat in excess of one second to signal the operator lay a lightingof the associated call lampCL-or isopened *for an interval slightly inexcess of five secondsto signal the operator by a lightingoif thedisconnect lamp Di ata ftimewhen :a-p'lug is in either of the jacksJi-andill.

:1. Ina telegraph :system wherein a switchboard is-wprovided forinterconnecting "and disconnecting two-way telegraph lines terminatingthereat, line circuits individual respectively to said lines, means,effiective when two lines are interconnected at said switchboard formutualtwo-way transmission, for controlling either eoncerned linecircuit from-its. individual' li-ne to transmit a signal to theiswitchboard indicative of such ind-inidual dine, and means forpreventing the other line circuit from responsively transmitting asignal to the switchboard; thereby preventing the .12 control exercisedover one line from causing; signal to be transmitted to the switchboardindicative .of the other line.

2. In a telegraph system wherein a switchboard is provided forinterconnecting and disconnecting two-way telegraph lines terminatingthereat formutua'l two-way transmission of signals betweeninterconnected lines, line circuits individual respectively to saidlines, means, effective when two lines are interconnected .at saidswitchboard, 'for controlling either concerned line circuit from itsindividual line to transmit a supervisory signal to the switchboardresponsive to a telegraph signal from its individual line and subject tosuch signal being of prolonged (duration, and means for preventing theother "line circuit Jfrom transmitting a similar supervisory signalresponsive to the'prolonged signal consequently transmitted over theother line.

'3. In a telegraph system wherein a switch board is provided forinterconnecting and disconnecting more than two two-way telegraph linesfor mutual two-way transmission of signals between any line and reach oftwo or more lines so interconnected therewith, line circuitsindividualrespectively to said lines, means, effective when lines areinterconnected at said switchboard, for controlling any concerned linecircuit from its individual line to transmit a supervisory signal to theswitchboard responsive to a telegraph signal from its individual lineand subject to' such Signal being of prolonged duration, and means forpreventing any other line circuit from transmitting a similarsupervisory signal responsive to the prolonged signal consequentlytransmitted over its individual line.

ORLANDO BLYHOLDER.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 2,206,663 Cramer et a1 July 21940 2252,6129 Hanley Aug. 12, 1941 2362,1941 Dodd Nov. 7, 19442;3fi9;578 Kinkead Feb. 13, 1945 2,387,246 Demonet Oct. 23, 1945

